diff --git a/playwright/_impl/_network.py b/playwright/_impl/_network.py
index 3e5b8fe89..229883657 100644
--- a/playwright/_impl/_network.py
+++ b/playwright/_impl/_network.py
@@ -18,7 +18,6 @@
 import json as json_utils
 import mimetypes
 import re
-from collections import defaultdict
 from pathlib import Path
 from types import SimpleNamespace
 from typing import (
@@ -1031,9 +1030,10 @@ def _on_close(self) -> None:
 class RawHeaders:
     def __init__(self, headers: HeadersArray) -> None:
         self._headers_array = headers
-        self._headers_map: Dict[str, Dict[str, bool]] = defaultdict(dict)
+        self._headers_map: Dict[str, Dict[str, bool]] = {}
         for header in headers:
-            self._headers_map[header["name"].lower()][header["value"]] = True
+            values = self._headers_map.setdefault(header["name"].lower(), {})
+            values[header["value"]] = True
 
     @staticmethod
     def _from_headers_dict_lossy(headers: Dict[str, str]) -> "RawHeaders":
@@ -1047,7 +1047,7 @@ def get(self, name: str) -> Optional[str]:
         return separator.join(values)
 
     def get_all(self, name: str) -> List[str]:
-        return list(self._headers_map[name.lower()].keys())
+        return list(self._headers_map.get(name.lower(), {}).keys())
 
     def headers(self) -> Dict[str, str]:
         result = {}
diff --git a/tests/async/test_network.py b/tests/async/test_network.py
index e88b24c24..b03490ef6 100644
--- a/tests/async/test_network.py
+++ b/tests/async/test_network.py
@@ -369,6 +369,9 @@ async def test_response_headers_should_work(page: Page, server: Server) -> None:
     assert response
     assert response.headers["foo"] == "bar"
     assert (await response.all_headers())["foo"] == "bar"
+    # Looking up a missing header must not add it.
+    assert await response.header_value("x-missing") is None
+    assert "x-missing" not in await response.all_headers()
 
 
 async def test_request_post_data_should_work(page: Page, server: Server) -> None: